The exemplary embodiment relates to the detection of wear particles in a lubrication fluid, such as oil. It finds particular application in connection with a microfluidic device for detecting or measuring an electrical property of the lubrication fluid which changes as the fluid becomes contaminated with wear particles.
Machine parts, such as aircraft engines and gear boxes in which components move relative to each other, are often lubricated with a lubricant oil to reduce wear. However, over time, small wear particles with sizes in the range of 1-10 microns (μm) are generated. When abnormal wear begins, larger particles, in the range of 10 to 50 microns are generated. The particle population and size of the particles tends to increase over time until eventually, a machine failure can result.
To monitor the change in lubricant wear particles, samples of the oil may be withdrawn from the machine at scheduled times and sent to a laboratory for analysis. A variety of off-line methods exist for measuring properties of lubricating fluids. For example, the suspended particles may be separated from the oil sample, e.g., by using a rotary particle depositor, and the amount of particulate matter contained in a given sample volume of oil is then quantified. Another method involves placing the sample in a container and creating a magnetic flux field using a sensing electromagnetic coil. The distortion of the flux field caused by the particle burden is then noted as a numerical Particle Quantifying (PQ) value (see U.S. Pat. No. 5,404,100). However, each of these methods takes time to generate wear information. As a result, critical failures of machines may occur even when samples are sent regularly for testing.
Ferrography is another method for lubricant debris analysis. However, the test procedure is very lengthy, and requires complicated setup and a skilled analyst (See, Roylance B. J., 2005, Tribology International, v. 38, pp. 857-862). Optical methods such as scattering counters are capable of detecting particles in oil. However, the accuracy of the optical approach is affected by particle properties (refractive index, shape, etc) and the existence of air bubbles, and is effective only for debris larger than 50 μm (See Khandakar G. and Jones G. R., 1993, Meas. Sci. Tech., v. 4, pp. 608). Magnetic inductive debris sensors have met some success but are limited to ferromagnetic debris larger than 100 μm (See, Campbell. P., 1991, Int. Condition Monitoring Conf. Proc., pp. 325-335). For example, U.S. Pat. No. 5,604,441 discloses a method and apparatus for detecting the degree of deterioration of a lubricating oil for an operating machine which includes a grid-like capacitive sensor that uses the lubricating oil as a dielectric medium. A magnetic field is imposed upon the oil to attract ferromagnetic wear particles into the vicinity of the sensor. Capacitance measurements are taken at periodic intervals at each of several magnet operational states for respective classification and analysis. The magnets are simultaneously de-energized for release of captured particles back in to an oil circulation stream and to clean the capacitative sensor grid of accumulated particulates. Such a method, however, is only applicable to ferromagnetic particles present in relatively high concentrations.
There remains a need for a method which permits in-situ testing of lubricants that allows a rapid response when wear particles reach a critical size or number and which is applicable to wear particles that do not have ferromagnetic properties.